Filters for separating solid particulate matter from a suspending gas find varied and wide application. These applications span such diverse uses as air conditioner filters, automobile air cleaners, air conveyor separator bags, vacuum cleaner bags, and the like. Filters of this nature use as the filtering means a fabric which is either a woven or a non-woven fabric. The woven fabric has the disadvantage (as opposed to the non-woven fabric) that it is relatively more expensive and the filtering efficiency (fineness of filtration at a given pressure drop) is lower. However, the woven fabric has the advantage of significantly greater tensile properties and may thus be used in more demanding applications, e.g. a "permanent" type filter. On the other hand, the non-woven fabric has the disadvantage (as opposed to the woven fabric) that the tensile properties are considerably less and, hence, the applications thereof are more limited. The non-woven fabric, however, has the advantages of lower cost and higher filtering efficiency.
For the foregoing reasons, filters have been generally divided into two groups, i.e., permanent filters and disposable filters. The woven fabrics have been generally used for the permanent filters, since these require the higher tensile characteristics, and the lower filtering efficiency is simply tolerated. The non-woven fabrics have been generally used for disposable filters, since these usually require the lower costs (with the higher filtering efficiency which further lowers cost) and the lower tensile characteristics are accepted for one-time (non-cleanable) use.
Examples of permanent filters using woven fabrics as the filtering element are air conveyor separator bags, industrial vacuum cleaner bags, and the like, while examples of disposable filters using non-woven fabrics as the filtering element are furnace filters, air conditioner filters, automobile engine filters and home vacuum cleaner bags. The non-woven fabrics most often used are not a textile fabric but a vitreous or cellulosic fabric such as spun glass (or slag) or paper.
The art has sought to overcome the difficulties of the non-woven fabrics by using special non-woven fabrics, referred to as needled textile fabrics. The needled fabrics are considerably improved in regard to the tensile properties, so as to allow the use of these fabrics in more demanding applications. However, the needling tends to increase the overall pressure drop while also tending to decrease the fineness of filtration--hence lowering the filtering efficiency. Further, in some applications the face surface of these needled fabrics tends to "blind" with repeated use. In other applications, leakage of the solids to be filtered through needle holes and poorly needled regions makes these fabrics unacceptable. Thus, needled fabrics as filtering fabrics have been commercially accepted in only special applications.